Waveguide

ABSTRACT

A waveguide has a front and a rear surface, the waveguide for a display system and arranged to guide light from a light engine onto an eye of a user to make an image visible to the user, the light guided through the waveguide by reflection at the front and rear surfaces. A first portion of the front or rear surface has a structure which causes light to change phase upon reflection from the first portion by a first amount. A second portion of the same surface has a different structure which causes light to change phase upon reflection from the second portion by a second amount different from the first amount. The first portion is offset from the second portion by a distance which substantially matches the difference between the second amount and the first amount.

BACKGROUND

Display systems can used to make a desired image visible to a user(viewer). Wearable display systems can be embodied in a wearable headsetwhich is arranged to display an image within a short distance from ahuman eye. Such wearable headsets are sometimes referred to as headmounted displays, and are provided with a frame which has a centralportion fitting over a user's (wearer's) nose bridge and left and rightsupport extensions which fit over a user's ears. Optical components arearranged in the frame so as to display an image within a few centimetersof the user's eyes. The image can be a computer generated image on adisplay, such as a micro display. The optical components are arranged totransport light of the desired image which is generated on the displayto the user's eye to make the image visible to the user. The display onwhich the image is generated can form part of a light engine, such thatthe image itself generates collimated lights beams which can be guidedby the optical component to provide an image visible to the user.

Different kinds of optical components have been used to convey the imagefrom the display to the human eye. These can include lenses, mirrors,optical waveguides, holograms and diffraction gratings, for example. Insome display systems, the optical components are fabricated using opticsthat allows the user to see the image but not to see through this opticsat the “real world”. Other types of display systems provide view throughthis optics so that the generated image which is displayed to the useris overlaid onto a real world view. This is sometimes referred to asaugmented reality.

Waveguide-based display systems typically transport light from a lightengine to the eye via a TIR (Total Internal Reflection) mechanism in awaveguide (light guide). Such systems can incorporate diffractiongratings, which cause effective beam expansion so as to output expandedversions of the beams provided by the light engine. This means the imageis visible over a wider area when looking at the waveguide's output thanwhen looking at the light engine directly: provided the eye is within anarea such that it can receive some light from substantially all of theexpanded beams, the whole image will be visible to the user. Such anarea is referred to as an eye box.

To maintain image quality, the structure of the waveguide can beconfigured in various ways to mitigate distortion of the transportedlight.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Nor is theclaimed subject matter limited to implementations that solve any or allof the disadvantages noted in the background section.

According to a first aspect a waveguide has a front and a rear surface.The waveguide is for a display system and is arranged to guide lightfrom a light engine onto an eye of a user to make an image visible tothe user. The light is guided through the waveguide by reflection at thefront and rear surfaces. A first portion of the front or rear surfacehas a structure which causes light to change phase upon reflection fromthe first portion by a first amount. A second portion of the samesurface has a different structure which causes light to change phaseupon reflection from the second portion by a second amount differentfrom the first amount. The first portion is offset from the secondportion by a distance which substantially matches the difference betweenthe second amount and the first amount.

According to a second aspect an image display system comprises a lightengine configured to generate an image and a waveguide having a frontand a rear surface. The waveguide is arranged to guide light from thelight engine onto an eye of a user to make the image visible to theuser, the light guided through the waveguide by reflection at the frontand rear surfaces. A first portion of the front or rear surface has astructure which causes light to change phase upon reflection from thefirst portion by a first amount. A second portion of the same surfacehas a different structure which causes light to change phase uponreflection from the second portion by a second amount different from thefirst amount. The first portion is offset from the second portion by adistance which substantially matches the difference between the secondamount and the first amount.

According to a third aspect a wearable image display system comprising:a headpiece; a light engine mounted on the headpiece and configured togenerate an image; and a waveguide located forward of an eye of a wearerin use. The waveguide has a front and a rear surface, and is arranged toguide light from the display onto the eye of the wearer to make theimage visible to the wearer, the light guided through the waveguide byreflection at the front and rear surfaces. A first portion of the frontor rear surface has a structure which causes light to change phase uponreflection from the first portion by a first amount. A second portion ofthe same surface has a different structure which causes light to changephase upon reflection from the second portion by a second amountdifferent from the first amount. The first portion is offset from thesecond portion by a distance which substantially matches the differencebetween the second amount and the first amount.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 shows a wearable display system;

FIG. 2A shows a plan view of part of the display system;

FIGS. 3A and 3B shows perspective and frontal view of an opticalcomponent;

FIG. 4A shows a schematic plan view of an optical component having asurface relief grating formed on its surface;

FIG. 4B shows a schematic illustration of the optical component of FIG.4A, shown interacting with incident light and viewed from the side;

FIG. 5A shows a schematic illustration of a straight binary surfacerelief grating, shown interacting with incident light and viewed fromthe side;

FIG. 5B shows a schematic illustration of a slanted binary surfacerelief grating, shown interacting with incident light and viewed fromthe side;

FIG. 5C shows a schematic illustration of an overhanging triangularsurface relief grating, shown interacting with incident light and viewedfrom the side;

FIG. 6 shows a close up view of part of an incoupling zone of an opticalcomponent;

FIG. 7A shows a perspective view of a part of a display system;

FIG. 7B shows a plan view of individual pixels of a display;

FIGS. 7C and 7D show plan and frontal views of a beam interacting withan optical component;

FIG. 7E shows a frontal view of an optical component performing beamexpansion;

FIG. 7F shows a plan view of an optical component performing beamexpansion;

FIG. 7G is a plan view of a curved optical component;

FIGS. 8A and 8B are plan and frontal views of a part of an opticalcomponent;

FIG. 9A shows a perspective view of beam reflection within a fold zoneof a waveguide;

FIG. 9B illustrates a beam expansion mechanism;

FIGS. 10A and 10B show plan and side views of a part of a waveguidehaving optical elements which are not offset from one another, and FIG.10C shows a phase distribution for the waveguide of FIGS. 10A and 10B;

FIG. 11A shows a side view of a part of a waveguide having opticalelements which are not offset from one another and which exhibitapodization, and FIG. 11B shows a phase distribution for the waveguideof FIG. 11A.

FIG. 12A shows a side view of a part of a first optical component havingoffset optical elements, and FIG. 12B shows a phase distribution for thesecond waveguide of FIG. 12A;

FIGS. 13A and 13B show plan and side views of a part of a secondwaveguide having offset optical elements, and FIG. 13C shows a phasedistribution for the second waveguide of FIGS. 13A and 13B;

FIG. 14 shows graphs of simulated performance data for the waveguide ofFIGS. 11A and 11B;

FIG. 15 shows graphs of simulated performance data for the firstwaveguide of FIG. 12A;

FIG. 16 shows a flow chart for a microfabrication process formanufacturing optical components or masters;

FIG. 17A shows an exemplary optical component having certaincharacteristics which may impact on image quality;

FIG. 17B shows an exposure set-up which could be used in making theoptical component of FIG. 17A;

FIG. 18 shows a graph of MTF as function of gap width for an exemplarywaveguide.

DETAILED DESCRIPTION

Typically, a waveguide based display system comprises an image source,e.g. a projector, waveguide(s) and various optical elements (e.g.diffraction gratings) imprinted on the waveguide surfaces.

FIGS. 10A and 10B show side and plan views of part of an opticalwaveguide 10 a having diffractive optical elements O1, O2 (which arediffraction gratings) imprinted on the top of the waveguide's surface.The first grating O1 has a depth h1 and the second grating O2 has adepth h2. An expanded side view of the optical elements O1, O2 is shownat the top of FIG. 10A. Each optical element is formed of a series ofgrooves in the surface of the waveguide 10 a of depth h1, h2≠h1respectively as measured normal to the waveguide. The depths h1, h2 areconstant across the whole of the gratings O1, O2 in this example.

The first and second elements O1, O2 are used, for example, to couplelight emitted by the image source into and out of the waveguide, and/orfor manipulation of its spatial distribution within the waveguide. Whilebeing necessary for the operation of the display system, the opticalelements O1, O2 can also cause unwanted distortions on the phase frontof the light field as it travels through the waveguide. In particular,phase distortions may be created when the wavefront meets the edges ofthe optical elements O1, O2. Elements may also change the amplitude ofthe field differently, i.e. there will be amplitude variation as well.However, in terms of image quality the phase distortion is much moresevere and matching of amplitude of the field portions is not requiredto achieve acceptable image quality.

The optical elements O1 and O2 are separated by a blank surface regionB, which is substantially non-diffractive (i.e. which interacts withlight substantially in accordance with Snell's law and the law ofreflection). Portions of the wavefront that are totally internallyreflected from the blank surface region B of the light guide experiencea different phase retardation than portions that are reflected from theoptical elements O1, O2. A ray R0 change phase upon total internalreflection from the (or any other) blank surface region B by an amountφ0 which depends on the polarization of the incident light. A ray R1change phase upon reflection from the first optical element O1 by anamount φ1=φ0−Δφ1. A ray R2 change phase upon reflection from the secondoptical component O2 by an amount φ2=φ0−φ2. This is illustrated in thephase distribution of FIG. 10C, which shows the phase of the rays R1,R0, R2 after reflection from the first optical element O1, the blanksurface B, and the second optical element O2 respectively.

Generally, gratings and TIR change the phase of polarization componentsdifferently, i.e. there is polarization rotation as well. As will beapparent, the description of the preceding paragraph is a simplificationto aid illustration of the distortion mechanism.

Note the term “reflected” as it is used herein includes reflectivelydiffracted light e.g. as created by a reflective or partially reflectivediffraction grating. Both zero and higher order modes can experiencephase retardation. In general, polarization of reflected higher ordermodes as well as 0th order mode can be rotated or turned into/out ofelliptical polarization etc.

Such phase jumps result in diffractive beam spreading and thus loss ofimage sharpness. One method to reduce the effect of edge diffractionwould be to use apodization. Generally this means using some form ofsmoothing to turn sharp edges into more continuously varying shapes. Thesmoothing can be done through various means. In the case of gratings thedepth of the grating structure, or more generally any other profileparameter, could be varied smoothly between two regions. An exemplarywaveguide 10 b exhibiting apodization is shown in FIG. 11A. Thewaveguide 10 b has first and second optical elements O1′, O2′ which areequivalent to the optical component O1, O2 of the waveguide 10 a but forthe fact that the depth of O1′, O2′ gradually decreases to zerothroughout first and second apodization regions A1, A2 respectively,which are adjacent a blank region B that separates the optical elementsO1′ and O2′. This results in gradually varying phase in the apodizedarea, as shown in the phase distribution of FIG. 11B. As can be seen inFIG. 11B, the amount by which the phase of rays changes due toreflection in the apodization regions A1, A2 varies as a function oflocation across the apodization regions A1, A2, with rays reflectedcloser to the blank region B exhibiting phase changes which are close tothat exhibited by rays reflected in the blank region B itself. Whilstthis can in some cases lead to reduced strength edge diffraction anddiffractive beam spreading, apodization can have other undesired effectse.g. it can reduce the efficiency of the grating near its edges.

The present disclosure provides a means to reduce phase distortionscaused by diffractive optical elements imprinted on the surface of thelight guide. In particular, the effect of the grating edge on thewavefront is removed by adding a suitable height offset to a gratingand/or to the blank surface (or other grating) next to it. The offset isselected so that the total phase retardance for rays that are reflectedfrom the offset grating is equal to the phase retardance of rays thatare totally internally reflected from the blank surface of the waveguide(or that are reflected from the other grating).

As compared with the method of using apodization, the method of thepresent disclosure allows for improved reduction of phase distortions ascompared with apodization. This is achieved while maintaining otherdesired properties of the gratings, e.g. gratings can be optimized forefficiency over the entire surface area of the gratings, including atthe edges of the grating, by for instance maintaining a desired depthprofile right up to the edges of the grating.

This is described in detail below. First, a context in which thewaveguides of the present disclosure can be used will be described.

FIG. 1 is a perspective view of a head mounted display. The head mounteddisplay comprises a headpiece, which comprises a frame 2 having acentral portion 4 intended to fit over the nose bridge of a wearer, anda left and right supporting extension 6,8 which are intended to fit overa user's ears. Although the supporting extensions are shown to besubstantially straight, they could terminate with curved parts to morecomfortably fit over the ears in the manner of conventional spectacles.

The frame 2 supports left and right optical components, labelled 10L and10R, which are waveguides. For ease of reference herein an opticalcomponent 10 (which is a waveguide) will be considered to be either aleft or right component, because the components are essentiallyidentical apart from being mirror images of each other. Therefore, alldescription pertaining to the left-hand component also pertains to theright-hand component. The optical components will be described in moredetail later with reference to FIG. 3. The central portion 4 houses alight engine which is not shown in FIG. 1 but which is shown in FIG. 2.

FIG. 2 shows a plan view of a section of the top part of the frame ofFIG. 1. Thus, FIG. 2 shows the light engine 13 which comprises a microdisplay 15 and imaging optics 17 in the form of a collimating lens 20.The light engine also includes a processor which is capable ofgenerating an image for the micro display. The micro display can be anytype of light of image source, such as liquid crystal on silicon (LCOS)displays (LCD's), transmissive liquid crystal displays (LCD), matrixarrays of LED's (whether organic or inorganic) and any other suitabledisplay. The display is driven by circuitry which is not visible in FIG.2 which activates individual pixels of the display to generate an image.The substantially collimated light, from each pixel, falls on an exitpupil 22 of the light engine 13. At exit pupil 22, collimated lightbeams are coupled into each optical component, 10L, 10R into arespective in-coupling zone 12L, 12R provided on each component. Thesein-coupling zones are clearly shown in FIG. 1, but are not readilyvisible in FIG. 2. In-coupled light is then guided, through a mechanismthat involves diffraction and TIR, laterally of the optical component ina respective intermediate (fold) zone 14L, 14R, and also downward into arespective exit zone 16L, 16R where it exits the component 10 towardsthe users' eye. The zones 14L, 14R, 16L and 16R are shown in FIG. 1.These mechanisms are described in detail below. FIG. 2 shows a user'seye (right or left) receiving the diffracted light from an exit zone(16L or 16R). The output beam OB to a user's eye is parallel with theincident beam IB. See, for example, the beam marked IB in FIG. 2 and twoof the parallel output beams marked OB in FIG. 2. The optical component10 is located between the light engine 13 and the eye i.e. the displaysystem configuration is of so-called transmissive type.

Other headpieces are also within the scope of the subject matter. Forinstance, the display optics can equally be attached to the users headusing a head band, helmet or other fit system. The purpose of the fitsystem is to support the display and provide stability to the displayand other head borne systems such as tracking systems and cameras. Thefit system will also be designed to meet user population inanthropometric range and head morphology and provide comfortable supportof the display system.

Beams from the same display 15 may be coupled into both components 10L,10R so that an image is perceived by both eyes from a single display, orseparate displays may be used to generate different images for each eyee.g. to provide a stereoscopic image. In alternative headsets, lightengine(s) may be mounted at one or both of left and right portions ofthe frame—with the arrangement of the incoupling, fold and exit zones12, 14, 16 flipped accordingly.

The optical component 10 is substantially transparent such that a usercan not only view the image from the light engine 13, but also can viewa real world view through the optical components.

The optical component 10 has a refractive index n which is such thattotal internal reflection takes place guiding the beam from theincoupling zone along the intermediate expansion zone 14, and downtowards the exit zone 16.

FIGS. 3A and 3B show an optical component in more detail.

FIG. 3A shows a perspective view of an optical component 10. The opticalcomponent is flat in that the front and rear portions of its surface aresubstantially flat (front and rear defined from the viewpoint of thewearer, as indicated by the location of the eye in FIG. 3A). The frontand rear portions of the surface are parallel to one another. Theoptical component 10 lies substantially in a plane (xy-plane), with thez axis (referred to as the “normal”) directed towards the viewer fromthe optical component 10. The incoupling, fold and exit zones 12, 14 and16 are shown, each defined by respective surface modulations 52, 46 and56 on the surface of the optical component, which are on the rear of thewaveguide from a viewpoint of the wearer. Each of the surfacemodulations 52, 46, 56 forms a respective surface relief grating (SRG),the nature of which will be described shortly. Instead of the SRGs,holograms could be used providing the same optical function as the SRGs.

As shown in the plan view of FIG. 3B, the fold zone has a horizontalextent W2 (referred to herein as the “length” of the expansion zone) inthe lateral (x) direction and an extent H2 in the y direction (referredto herein as the “length” of the expansion zone) which increases fromthe inner edge of the optical component to its outer edge in the lateraldirection along its width W2. The exit zone has a horizontal extent W3(length of the exit zone) and y-direction extent H3 (width of the exitzone) which define the size of the eye box, which size is independent ofthe imaging optics in the light engine. The incoupling and fold SRGs 52,54 have a relative orientation angle A, as do the fold and exit SRGs 54,56 (note the various dotted lines superimposed on the SRGs 52, 54, 56denote directions perpendicular to the grating lines of those SRGs).

The incoupling and fold zones 12, 14 are substantially contiguous inthat they are separated by at most a narrow border zone 18 which has awidth W as measured along (that is, perpendicular to) a common border 19that divides the border zone 18. The common border 19 is arcuate(substantially semi-circular in this example), the incoupling and foldregions 12, 14 having edges which are arcuate (substantiallysemi-circular) along the common border 19. The edge of incoupling region12 is substantially circular overall.

Principles of the diffraction mechanisms which underlie operation of thehead mounted display described herein will now be described withreference to FIGS. 4A and 4B.

The optical components described herein interact with light by way ofreflection, refractions and diffraction. Diffraction occurs when apropagating wave interacts with a structure, such as an obstacle orslit. Diffraction can be described as the interference of waves and ismost pronounced when that structure is comparable in size to thewavelength of the wave. Optical diffraction of visible light is due tothe wave nature of light and can be described as the interference oflight waves. Visible light has wavelengths between approximately 390 and700 nanometers (nm) and diffraction of visible light is most pronouncedwhen propagating light encounters structures of a similar scale e.g. oforder 100 or 1000 nm in scale.

One example of a diffractive structure is a periodic (substantiallyrepeating) diffractive structure. Herein, a “diffraction grating” meansany (part of) an optical component which has a periodic diffractivestructure. Periodic structures can cause diffraction of light, which istypically most pronounced when the periodic structure has a spatialperiod of similar size to the wavelength of the light. Types of periodicstructures include, for instance, surface modulations on the surface ofan optical component, refractive index modulations, holograms etc. Whenpropagating light encounters the periodic structure, diffraction causesthe light to be split into multiple beams in different directions. Thesedirections depend on the wavelength of the light thus diffractionsgratings cause dispersion of polychromatic (e.g. white) light, wherebythe polychromatic light is split into different coloured beamstravelling in different directions.

When the periodic structure is on the surface of an optical component,it is referred to a surface grating. When the periodic structure is dueto modulation of the surface itself, it is referred to as a surfacerelief grating (SRG). An example of a SRG is uniform straight grooves ina surface of an optical component that are separated by uniform straightgroove spacing regions. Groove spacing regions are referred to herein as“lines”, “grating lines” and “filling regions”. The nature of thediffraction by a SRG depends both on the wavelength of light incident onthe grating and various optical characteristics of the SRG, such as linespacing, groove depth and groove slant angle. An SRG can be fabricatedby way of a suitable microfabrication process, which may involve etchingof and/or deposition on a substrate to fabricate a desired periodicmicrostructure on the substrate to form an optical component, which maythen be used as a production master such as a mould for manufacturingfurther optical components.

An SRG is an example of a Diffractive Optical Element (DOE). When a DOEis present on a surface (e.g. when the DOE is an SRG), the portion ofthat surface spanned by that DOE is referred to as a DOE area.

FIGS. 4A and 4B show from the top and the side respectively part of asubstantially transparent optical component 10 having an outer surfaceS. At least a portion of the surface S exhibits surface modulations thatconstitute a SRG 44 (e.g. 52, 54, 56), which is a microstructure. Such aportion is referred to as a “grating area”. The modulations comprisegrating lines which are substantially parallel and elongate(substantially longer than they are wide), and also substantiallystraight in this example (though they need not be straight in general).

FIG. 4B shows the optical component 10, and in particular the SRG 44,interacting with an incoming illuminating light beam I that is inwardlyincident on the SRG 44. The light I is white light in this example, andthus has multiple colour components. The light I interacts with the SRG44 which splits the light into several beams directed inwardly into theoptical component 10. Some of the light I may also be reflected backfrom the surface S as a reflected beam R0. A zero-order mode inward beamT0 and any reflection R0 are created in accordance with the normalprinciples of diffraction as well as other non-zero-order (±n-order)modes (which can be explained as wave interference). FIG. 4B showsfirst-order inward beams T1, T−1; it will be appreciated thathigher-order beams may or may not also be created depending on theconfiguration of the optical component 10. Because the nature of thediffraction is dependent on wavelength, for higher-order modes,different colour components (i.e. wavelength components) of the incidentlight I are, when present, split into beams of different colours atdifferent angles of propagation relative to one another as illustratedin FIG. 4B.

FIGS. 5A-5C are close-up schematic cross sectional views of differentexemplary SRGs 44 a-44 c (collectively referenced as 44 herein) that maybe formed by modulation of the surface S of the optical component 10(which is viewed from the side in these figures). Light beams aredenoted as arrows whose thicknesses denote approximate relativeintensity (with higher intensity beams shown as thicker arrows).

FIG. 5A shows an example of a straight binary SRG 44 a. The straightbinary SRG 44 a is formed of a series of grooves 7 a in the surface Sseparated by protruding groove spacing regions 9 a which are alsoreferred to herein as “filling regions”, “grating lines” or simply“lines”. The SRG 44 a has a spatial period of d (referred to as the“grating period”), which is the distance over which the modulations'shape repeats and which is thus the distance between adjacentlines/grooves. The grooves 7 a have a depth h and have substantiallystraight walls and substantially flat bases. The filling regions have aheight h and a width that is substantially uniform over the height h ofthe filling regions, labelled “w” in FIG. 2A (with w being some fractionf of the period: w=f*d).

For a straight binary SRG, the walls are substantially perpendicular tothe surface S. The SRG 44 a causes symmetric diffraction of incidentlight I that is entering perpendicularly to the surface, in that each+n-order mode beam (e.g. T1) created by the SRG 4 a has substantiallythe same intensity as the corresponding −n-order mode beam (e.g. T−1),typically less than about one fifth (0.2) of the intensity of theincident beam I.

FIG. 5B shows an example of a slanted binary SRG 44 b. The slantedbinary SRG 44 b is also formed of grooves, labelled 7 b, in the surfaceS having substantially straight walls and substantially flat basesseparated by lines 9 b of width w. However, in contrast to the straightSRG 44 a, the walls are slanted by an amount relative to the normal,denoted by the angle β in FIG. 25B. The grooves 7 b have a depth h asmeasured along the normal. Due to the asymmetry introduced by thenon-zero slant, ±n-order mode inward beams travelling away from theslant direction have greater intensity that their ∓n-order modecounterparts (e.g. in the example of FIG. 2B, the T1 beam is directedaway from the direction of slant and has usually greater intensity thanthe T−1 beam, though this depends on e.g. the grating period d); byincreasing the slant by a sufficient amount, those ∓n counterparts canbe substantially eliminated (i.e. to have substantially zero intensity).The intensity of the T0 beam is typically also very much reduced by aslanted binary SRG such that, in the example of FIG. 5B, the first-orderbeam T1 typically has an intensity of at most about four fifths (0.8)the intensity of the incident beam I but this is highly dependent onwavelength and incident angle.

The binary SRGs 44 a and 44 b can be viewed as spatial waveformsembedded in the surface S that have a substantially square wave shape(with period d). In the case of the SRG 44 b, the shape is a skewedsquare wave shape skewed by β.

FIG. 5C shows an example of an overhanging triangular SRG 44 c which isa special case of an overhanging trapezoidal SRG. The triangular SRG 44c is formed of grooves 7 c in the surface S that are triangular in shape(and which thus have discernible tips) and which have a depth h asmeasured along the normal. Filling regions 9 c take the form oftriangular, tooth-like protrusions (teeth), having medians that make anangle β with the normal (β being the slant angle of the SRG 44 c). Theteeth have tips that are separated by d (which is the grating period ofthe SRG 44 c), a width that is w at the base of the teeth and whichnarrows to substantially zero at the tips of the teeth. For the SRG ofFIG. 44c , w≈d, but generally can be w<d. The SRG is overhanging in thatthe tips of the teeth extend over the tips of the grooves. It ispossible to construct overhanging triangular SRGs that substantiallyeliminate both the transmission-mode T0 beam and the ∓n-mode beams,leaving only ±n-order mode beams (e.g. only T1). The grooves have wallswhich are at an angle γ to the median (wall angle).

The SRG 44 c can be viewed as a spatial waveform embedded in S that hasa substantially triangular wave shape, which is skewed by β.

Other SRGs are also possible, for example other types of trapezoidalSRGs (which may not narrow in width all the way to zero), sinusoidalSRGs etc. Such other SRGs also exhibit depth h, linewidth w, slant angleβ and wall angles γ which can be defined in a similar manner to FIG.5A-C.

In the present display system, d is typically between about 250 and 500nm, and h between about 30 and 400 nm. The slant angle β is typicallybetween about 0 and 45 degrees (such that slant direction is typicallyelevated above the surface S by an amount between about 45 and 90degrees).

An SRG has a diffraction efficiency defined in terms of the intensity ofdesired diffracted beam(s) (e.g. T1) relative to the intensity of theilluminating beam I, and can be expressed as a ratio η of thoseintensities. As will be apparent from the above, slanted binary SRGs canachieve higher efficiency (e.g. 4 b—up to η≈0.8 if T1 is the desiredbeam) than non-slanted SRGs (e.g. 44 a—only up to about η≈0.2 if T1 isthe desired beam). With overhanging triangular SRGs, it is possible toachieve near-optimal efficiencies of η≈1.

FIG. 6 shows the incoupling SRG 52 with greater clarity, including anexpanded version showing how the light beam interacts with it. FIG. 6shows a plan view of the optical component 10. The light engine 13provides beams of collimated light, one of which is shown (correspondingto a display pixel). That beam falls on the incoupling SRG 52 and thuscauses total internal reflection of the beam in the component 10. Theintermediate grating 14 directs versions of the beams down to the exitgrating 16, which causes diffraction of the image onto the user's eye.The operation of the grating 12 is shown in more detail in the expandedportion which shows rays of the incoming light beam coming in from theleft and denoted I and those rays being diffracted so as to undergo TIRin the optical component 10. The grating in FIG. 6 is of the type shownin FIG. 5B but could also be of the type shown in FIG. 5C or some otherslanted grating shape.

Optical principles underlying certain embodiments will now be describedwith reference to FIGS. 7A-9B.

Collimating optics of the display system is arranged to substantiallycollimate an image on a display of the display system into multipleinput beams. Each beam is formed by collimating light from a respectiveimage point, that beam directed to the incoupling grating in a uniqueinward direction which depends on the location of that point in theimage. The multiple input beams thus form a virtual version of theimage. The intermediate and exit grating have widths substantiallylarger than the beams' diameters. The incoupling grating is arranged tocouple each beam into the intermediate grating, in which that beam isguided onto multiple splitting regions of the intermediate grating in adirection along the width of the intermediate grating. The intermediategrating is arranged to split that beam at the splitting regions toprovide multiple substantially parallel versions of that beam. Thosemultiple versions are coupled into the exit grating, in which themultiple versions are guided onto multiple exit regions of the exitgrating. The exit regions lie in a direction along the width of the exitgrating. The exit grating is arranged to diffract the multiple versionsof that beam outwardly, substantially in parallel and in an outwarddirection which substantially matches the unique inward direction inwhich that beam was incoupled. The multiple input beams thus causemultiple exit beams to exit the waveguide which form substantially thesame virtual version of the image.

FIG. 7a shows a perspective view of the display 15, imaging optics 17and incoupling SRG 52. Different geometric points on the region of thedisplay 15 on which an image is displayed are referred to herein asimage points, which may be active (currently emitting light) or inactive(not currently emitting light). In practice, individual pixels can beapproximated as image points.

The imaging optics 17 can typically be approximated as a principal plane(thin lens approximation) or, in some cases, more accurately as a pairof principal planes (thick lens approximation) the location(s) of whichare determined by the nature and arrangement of its constituent lenses.In these approximations, any refraction caused by the imaging optics 17is approximated as occurring at the principal plane(s). To avoidunnecessary complication, principles of various embodiments will bedescribed in relation to a thin lens approximation of the imaging optics17, and thus in relation to a single principal plane labelled 31 in FIG.7a , but it will be apparent that more complex imaging optics that donot fit this approximation still can be utilized to achieve the desiredeffects.

The imaging optics 17 has an optical axis 30 and a front focal point,and is positioned relative to the optical component 10 so that theoptical axis 30 intersects the incoupling SRG 52 at or near thegeometric centre of the incoupling SRG 52 with the front focal pointlying substantially at an image point X₀ on the display (that is, lyingin the same plane as the front of the display). Another arbitrary imagepoint X on the display is shown, and principles underlying variousembodiments will now be described in relation to X without loss ofgenerality. In the following, the terminology “for each X” or similar isused as a convenient shorthand to mean “for each image point (includingX)” or similar, as will be apparent in context.

When active, image points—including the image point labelled X andX₀—act as individual illumination point sources from which lightpropagates in a substantially isotropic manner through the half-spaceforward of the display 15. Image points in areas of the image perceivedas lighter emit light of higher intensity relative to areas of the imageperceived as darker. Image points in areas perceived as black emit no oronly very low intensity light (inactive image points). The intensity ofthe light emitted by a particular image point may change as the imagechanges, for instance when a video is displayed on the display 15.

Each active image point provides substantially uniform illumination of acollimating area A of the imaging optics 17, which is substantiallycircular and has a diameter D that depends on factors such as thediameters of the constituent lenses (typically D is of order 1-10 mm).This is illustrated for the image point X in FIG. 7a , which shows howany propagating light within a cone 32(X) from X is incident on thecollimating area A. The imaging optics collimates any light 32(X)incident on the collimating area A to form a collimated beam 34(X) ofdiameter D (input beam), which is directed towards the incouplinggrating 52 of the optical component 10. The beam 34(X) is thus incidenton the incoupling grating 52. A shielding component (not shown) may bearranged to prevent any un-collimated light from outside of the cone32(X) that is emitted from X from reaching the optical component 10.

The beam 34(X) corresponding to the image point X is directed in aninward propagation direction towards the incoupling SRG 52, which can bedescribed by a propagation vector {circumflex over (k)}_(in) (X)(herein, bold typeface is used to denote 3-dimensional vectors, withhats on such vectors indicating denoting a unit vector). The inwardpropagation direction depends on the location of X in the image and,moreover, is unique to X. That unique propagation direction can beparameterized in terms of an azimuthal angle φ_(in)(X) (which is theangle between the x-axis and the projection of {circumflex over(k)}_(in) (X) in the xy-plane) and a polar angle θ_(in)(X)(which is theangle between the z-axis and {circumflex over (k)}_(in) (P) as measuredin the plane in which both the z-axis and {circumflex over (k)}_(in) (X)lie—note this is not the xz-plane in general). The notation φ_(in)(X),θ_(in)(X) is adopted to denote the aforementioned dependence on X; asindicated φ_(in)(X), θ_(in)(X) are unique to that X. Note that, herein,both such unit vectors and such polar/azimuthal angle pairsparameterizing such vectors are sometimes referred herein to as“directions” (as the latter represent complete parameterizationsthereof), and that sometimes azimuthal angles are referred to inisolation as xy-directions for the same reason. Note further that“inward” is used herein to refer to propagation that is towards thewaveguide (having a positive z-component when propagation is towards therear of the waveguide as perceived by the viewer and a negativez-component when propagation is towards the front of the waveguide).

The imaging optics has a principle point P, which is the point at whichthe optical axis 30 intersects the principal plane 31 and whichtypically lies at or near the centre of the collimation area A. Theinward direction {circumflex over (k)}_(in) (X) and the optical axis 30have an angular separation β(X) equal to the angle subtended by X and X₀from P. β(X)=θ_(in)(X) if the optical axis is parallel to the z-axis(which is not necessarily the case).

As will be apparent, the above applies for each active image point andthe imaging optics is thus arranged to substantially collimate the imagewhich is currently on the display 15 into multiple input beams, eachcorresponding to and propagating in a unique direction determined by thelocation of a respective active image point (active pixel in practice).That is, the imaging optics 17 effectively converts each active pointsource X into a collimated beam in a unique inward direction {circumflexover (k)}_(in) (X). As will be apparent, this can be equivalently statedas the various input beams for all the active image points forming avirtual image at infinity that corresponds to the real image that iscurrently on the display 17. A virtual image of this nature is sometimesreferred to herein as a virtual version of the image (or similar).

The input beam corresponding to the image point X₀ (not shown) wouldpropagate parallel to the optical axis 30, towards or near the geometriccentre of the incoupling SRG 52.

As mentioned, in practice, individual pixels of the display 15 can beapproximated as single image points. This is illustrated in FIG. 7Bwhich is a schematic plan view showing the principal plane 31 and twoadjacent pixels Xa, Xb of the display 15, whose centres subtend an angleΔβ from the principal point P. Light emitted the pixels Xa, Xb whenactive is effectively converted into collimated beams 34(Xa), 34(Xb)having an angular separation equal to Δβ. As will be apparent, the scaleof the pixels Xa, Xb has been greatly enlarged for the purposes ofillustration.

The beams are highly collimated, having an angular range no greater thanthe angle subtended by an individual pixel from P (˜Δβ) e.g. typicallyhaving an angular range no more than about ½ milliradian. As will becomeapparent in view of the following, this increases the image quality ofthe final image as perceived by the wearer.

FIGS. 7C and 7D show schematic plan (xz) and frontal (yz) views of partof the optical component respectively. As indicated in these figures,the incoupling grating 52 causes diffraction of the beam 34(X) therebycausing a first (±1) order mode beam to propagate within the opticalcomponent 10 in a new direction {circumflex over (k)}(X) that isgenerally towards the fold SRG 54 (i.e. that has a positivex-component). The new direction {circumflex over (k)}(X) can beparameterized by azimuthal and polar angles φ(X)—where|φ(X)|≦|φ_(in)(X)| and θ(X)—where |θ(X)|>|θ_(in)(X)|—which are alsodetermined by the location of and unique to the image point X. Thegrating 52 is configured so that the first order mode is the onlysignificant diffraction mode, with the intensity of this new beam thussubstantially matching that of the input beam. As mentioned above, aslanted grating can be used to achieve this desired effect (the beam asdirected away from the incoupling SRG 52 would correspond, for instance,to beam T1 as shown in FIG. 4B or 4C). In this manner, the beam 34(X) iscoupled into the incoupling zone 12 of the optical component 10 in thenew direction {circumflex over (k)}(X).

The optical component has a refractive index n and is configured suchthat the polar angle θ(X) satisfies total internal reflection criteriagiven by:sin 0(X)>1/n for each X.  (1):As will be apparent, each beam input from the imaging optics 17 thuspropagates through the optical component 10 by way of total internalreflection (TIR) in a generally horizontal (+x) direction (offset fromthe x-axis by φ(X)<φ_(in)(X)). In this manner, the beam 34(X) is coupledfrom the incoupling zone 12 into the fold zone 14, in which itpropagates along the width of the fold zone 14.

FIG. 7E shows 10 a frontal (xy) view of the whole of the opticalcomponent 10, from a viewpoint similar to that of the wearer. Asexplained in more detail below, a combination of diffractive beamsplitting and total internal reflection within the optical component 10results in multiple versions of each input beam 34(X) being outwardlydiffracted from the exit SRG along both the length and the width of theexit zone 16 as output beams 38(X) in respective outward directions(that is, away from the optical component 10) that substantially matchthe respective inward direction {circumflex over (k)}_(in) (X) of thecorresponding input beam 34(X).

In FIG. 7E, beams external to the optical component 10 are representedusing shading and dotted lines are used to represent beams within theoptical component 10. Perspective is used to indicate propagation in thez-direction, with widening (resp. narrowing) of the beams in FIG. 7Erepresenting propagation in the positive (resp. negative) z direction;that is towards (resp. away from) the wearer. Thus, diverging dottedlines represent beams within the optical component 10 propagatingtowards the front wall of the optical component 10; the widest partsrepresent those beams striking the front wall of the optical component10, from which they are totally internally reflected back towards therear wall (on which the various SRGs are formed), which is representedby the dotted lines converging from the widest points to the narrowestpoints at which they are incident on the rear wall. Regions where thevarious beams are incident on the fold and exit SRGs are labelled S andE and termed splitting and exit regions respectively for reasons thatwill become apparent.

As illustrated, the input beam 34(X) is coupled into the waveguide byway of the aforementioned diffraction by the incoupling SRG 52, andpropagates along the width of the incoupling zone 12 by way of TIR inthe direction φ(X), ±θ(X) (the sign but not the magnitude of the polarangle changing whenever the beam is reflected). As will be apparent,this results in the beam 34(X) eventually striking the fold SRG at theleft-most splitting region S.

When the beam 34(X) is incident at a splitting region S, that incidentbeam 34(X) is effectively split in two by way of diffraction to create anew version of that beam 42(X) (specifically a −1 reflection mode beam)which directed in a specific and generally downwards (−y) directionφ′(X), ±θ′(X) towards the exit zone 16 due to the fold SRG 54 having aparticular configuration which will be described in due course, inaddition to a zero order reflection mode beam (specular reflectionbeam), which continues to propagate along the width of the beam in thesame direction φ(X), ±θ(X) just as the beam 34(X) would in the absenceof the fold SRG (albeit at a reduced intensity). Thus, the beam 34(X)effectively continues propagates along substantially the whole width ofthe fold zone 14, striking the fold SRG at various splitting regions S,with another new version of the beam (in the same specific downwarddirection φ′(X), ±θ′(X)) created at each splitting region S. As shown inFIG. 7E, this results in multiple versions of the beam 34(X) beingcoupled into the exit zone 16, which are horizontally separated so as tocollectively span substantially the width of the exit zone 16.

As also shown in FIG. 7E, a new version 42(X) of the beam as created ata splitting region S may itself strike the fold SRG during its downwardpropagation. This will result in a zero order mode being created whichcontinues to propagate generally downwards in the direction φ′(X),±θ′(X) and which can be viewed as continued propagation of that beam,but may also result in a non-zero order mode beam 40(X) (further newversion) being created by way of diffraction. However, any such beam40(X) created by way of such double diffraction at the same SRG willpropagate in substantially the same direction φ(X), ±θ(X) along thewidth of the fold zone 14 as the original beam 34(X) as coupled into theoptical component 10 (see below). Thus, notwithstanding the possibilityof multiple diffractions by the fold SRG, propagation of the variousversions of the beam 34(X) (corresponding to image point X) within theoptical component 10 is effectively limited to two xy-directions: thegenerally horizontal direction (φ(X), ±θ(X)), and the specific andgenerally downward direction (φ′(X), ±θ′(X)) that will be discussedshortly.

Propagation within the fold zone 14 is thus highly regular, with allbeam versions corresponding to a particular image point X substantiallyconstrained to a lattice like structure in the manner illustrated.

The exit zone 16 is located below the fold zone 14 and thus thedownward-propagating versions of the beam 42(X) are coupled into theexit zone 16, in which they are guided onto the various exit regions Eof the output SRG. The exit SRG 56 is configured so as, when a versionof the beam strikes the output SRG, that beam is diffracted to create afirst order mode beam directed outwardly from the exit SRG 56 in anoutward direction that substantially matches the unique inward directionin which the original beam 34(X) corresponding to image point X wasinputted. Because there are multiple versions of the beam propagatingdownwards that are substantially span the width of the exit zone 16,multiple output beams are generated across the width of the exit zone 16(as shown in FIG. 7E) to provide effective horizontal beam expansion.

Moreover, the exit SRG 56 is configured so that, in addition to theoutwardly diffracted beams 38(X) being created at the various exitregions E from an incident beam, a zero order diffraction mode beamcontinuous to propagate downwards in the same specific direction as thatincident beam. This, in turn, strikes the exit SRG at a lower exit zone16 s in the manner illustrated in FIG. 7E, resulting in both continuingzero-order and outward first order beams. Thus, multiple output beams38(X) are also generated across substantially the width of the exit zone16 to provide effective vertical beam expansion.

The output beams 38(X) are directed outwardly in outward directions thatsubstantially match the unique input direction in which the originalbeam 34(X) is inputted. In this context, substantially matching meansthat the outward direction is related to the input direction in a mannerthat enables the wearer's eye to focus any combination of the outputbeams 38(X) to a single point on the retina, thus reconstructing theimage point X (see below).

For a flat optical component (that is, whose front and rear surfaces liesubstantially parallel to the xy-plane in their entirety), the outputbeams are substantially parallel to one another (to at least within theangle Δβ subtended by two adjacent display pixels) and propagateoutwardly in an output propagation direction {circumflex over(k)}_(out)(X) that is parallel to the unique inward direction{circumflex over (k)}_(in) (X) in which the corresponding input beam34(X) was directed to the incoupling SRG 52. That is, directing the beam34(X) corresponding to the image point X to the incoupling SRG 52 in theinward direction {circumflex over (k)}_(in) (X) causes correspondingoutput beams 38(X) to be diffracted outwardly and in parallel from theexit zone 16, each in an outward propagation direction {circumflex over(k)}_(out)(X)={circumflex over (k)}_(in) (X) due to the configuration ofthe various SRGs (see below).

As will now be described with reference to FIG. 7F, this enables aviewer's eye to reconstruct the image when looking at the exit zone 16.FIG. 7F shows a plan (xz) view of the optical component 10. The inputbeam 34(X) is in coupled to the optical component 10 resulting inmultiple parallel output beams 38(X) being created at the various exitregions E in the manner discussed above. This can be equivalentlyexpressed at the various output beams corresponding to all the imagepoints forming the same virtual image (at infinity) as the correspondinginput beams.

Because the beams 38(X) corresponding to the image point X are allsubstantially parallel, any light of one or more of the beam(s) 38(X)which is received by the eye 37 is focussed as if the eye 37 wereperceiving an image at infinity (i.e. a distant image). The eye 37 thusfocuses such received light onto a single retina point, just as if thelight were being received from the imaging optics 17 directly, thusreconstructing the image point X (e.g. pixel) on the retina. As will beapparent, the same is true of each active image point (e.g. pixel) sothat the eye 37 reconstructs the whole image that is currently on thedisplay 15.

However, in contrast to receiving the image directly from the optics17—from which only a respective single beam 34(X) of diameter D isemitted for each X—the output beams 39(X) are emitted over asignificantly wider area i.e. substantially that of the exit zone 16,which is substantially larger than the area of the inputted beam (˜D²).It does not matter which (parts) of the beam(s) 38(X) the eye receivesas all are focused to the same retina point—e.g., were the eye 37 to bemoved horizontally (±x) in FIG. 7F, it is apparent that the image willstill be perceived. Thus, no adaptation of the display system isrequired for, say, viewers with different pupillary distances beyondmaking the exit zone 16 wide enough to anticipate a reasonable range ofpupillary distances: whilst viewers whose eyes are closer together willgenerally receive light from the side of the exit zone 16 nearer theincoupling zone 12 as compared with viewers whose eyes are furtherapart, both will nonetheless perceive the same image. Moreover, as theeye 27 rotates, different parts of the image are brought towards thecentre of the viewer's field of vision (as the angle of the beamsrelative to the optical axis of the eye changes) with the image stillremaining visible, thereby allowing the viewer to focus their attentionon different parts of the image as desired.

The same relative angular separation Δβ exhibited the input beamscorresponding any two adjacent pixels Xa, Xb is also exhibited by thecorresponding sets of output beams 38(Xa), 38(Xb)—thus adjacent pixelsare focused to adjacent retina points by the eye 37. All the variousversions of the beam remain highly collimated as they propagate throughthe optical component 10, preventing significant overlap of pixel imagesas focused on the retina, thereby preserving image sharpness.

It should be noted that FIGS. 7A-7G are not to scale and that inparticular beams diameters are, for the sake of clarity, generallyreduced relative to components such as the display 15 than wouldtypically be expected in practice.

The configuration of the incoupling SRG 52 will now be described withreference to FIGS. 8A and 8B, which show schematic plan and frontalviews of part of the fold grating 52. Note, in FIGS. 8A and 8B, beamsare represented by arrows (that is, their area is not represented) forthe sake of clarity.

FIG. 8A shows two image points XL, XR located at the far left and farright of the display 15 respectively, from which light is collimated bythe optics 17 to generate respective input beams 34(XL), 34(XR) ininward directions (θ_(in)(XL), φ_(in)(XL)), (θ_(in)(XR), φ_(in) (XR)).These beams are coupled into the optical component 10 by the incouplingSRG 52 as shown—the incoupled beams shown created at the incoupling SRG52 are first order (+1) mode beams created by way of diffraction of thebeams incident on the SRG 52. The beams 34(XL), 34(XR) as coupled intothe waveguide propagate in directions defined by the polar angles θ(XL),θ(XR).

FIG. 8B shows two image points XR1 and XR2 at the far top-right and farbottom-right of the display 15. Note in this figure dashed-dotted linesdenote aspects which are behind the optical component 10 (−z).Corresponding beams 34(XL), 34(XR) in directions within the opticalcomponent 10 with polar angles φ(XL), φ(XR).

Such angles θ(X), φ(X) are given by the (transmissive) gratingequations:n sin θ(X)sin φ(X)=sin θ_(in)(X)sin φ_(in)(X)  (2)

$\begin{matrix}{{n\;\sin\;{\theta(X)}\cos\;{\phi(X)}} = {{\sin\;{\theta_{in}(X)}\cos\;{\phi_{in}(X)}} + \frac{\lambda}{d_{1}}}} & (3)\end{matrix}$with the SRG 52 having a grating period d₁, the beam light having awavelength λ, and n the refractive index of the optical component.

It is straightforward to show from (2), (3) that θ(XL)=θ_(max) andθ(XR)=θ_(min) i.e. that any beam as coupled into the component 10propagates with an initial polar angle in the range [θ(XR), θ(XL)]; andthat φ(XR2)=φ_(max) and φ(XR1)=φ_(min) (≈−φ_(max) in this example) i.e.that any beam as coupled into the component initially propagates with anazimuthal angle in the range [φ(XR1), φ(XR2)](≈[−φ(XR2), φ(XR2)]).

The configuration of the fold SRG 54 will now be described withreferences to FIGS. 9A-9B. Note, in FIGS. 9A and 9B, beams are againrepresented by arrows, without any representation of their areas, forthe sake of clarity. In these figures, dotted lines denote orientationsperpendicular to the fold SRG grating lines, dashed lines orientationsperpendicular to the incoupling SRG grating lines, and dash-dotted linesorientations perpendicular to the exit SRG grating lines.

FIG. 9A shows a perspective view of the beam 34(X) as coupled into thefold zone 14 of the optical component 10, having been reflected from thefront wall of the optical component 10 and thus travelling in thedirection (φ(X), −θ(X)) towards the fold SRG 54. A dotted line (whichlies perpendicular to the fold SRG grating lines) is shown to representthe orientation of the fold SRG.

The fold SRG 54 and incoupling SRG 52 have a relative orientation angleA (which is the angle between their respective grating lines). The beamthus makes an angle A+φ(X) (see FIG. 9B) with the fold SRG grating linesas measured in the xy-plane. The beam 34 is incident on the fold SRG 54,which diffracts the beam 34 into different components. A zero orderreflection mode (specular reflection) beam is created which continues topropagate in the direction (φ(X), +θ(X)) just as the beam 34(X) woulddue to reflection in the absence of the fold SRG 54 (albeit at a reducedintensity). This specular reflection beam can be viewed as effectively acontinuation of the beam 34(X) and for this reason is also labelled34(X). A first order (−1) reflection mode beam 42(X) is also createdwhich can be effectively considered a new version of the beam.

As indicated, the new version of the beam 42(X) propagates in a specificdirection (φ′(X), θ′(X)) which is given by the known (reflective)grating equations:n sin θ′(X)sin(A+φ′(X))=n sin θ(X)sin(A+φ(X))  (4)

$\begin{matrix}{{n\;\sin\;{\theta^{\prime}(X)}\cos\;\left( {A + {\phi^{\prime}(X)}} \right)} = {{n\;\sin\;{\theta(X)}\cos\;\left( {A + {\phi(X)}} \right)} - \frac{\lambda}{d_{2}}}} & (5)\end{matrix}$where the fold SRG has a grating period d₂, the beam light has awavelength λ and n is the refractive index of the optical component 10.

As shown in FIG. 9B, which shows a schematic frontal view of the opticalcomponent 10, the beam 34(X) is coupled into the incoupling zone 12 withazimuthal angle φ(X) and thus makes an xy-angle φ(X)+A the fold SRG 54.

A first new version 42 a(X) (−1 mode) of the beam 34(X) is created whenit is first diffracted by the fold SRG 54 and a second new version 42b(X) (−1 mode) when it is next diffracted by the fold SRG 54 (and soon), which both propagate in xy-direction φ′(X). In this manner, thebeam 34(X) is effectively split into multiple versions, which arehorizontally separated (across the width of the fold zone 14). These aredirected down towards the exit zone 16 and thus coupled into the exitzone 16 (across substantially the width of the exit zone 16 due to thehorizontal separation). As can be see, the multiple versions are thusincident on the various exit regions (labelled E) of the exit SRG 56,which lie along the width of the exit zone 16.

These new, downward (−y)-propagating versions may themselves meet thefold SRG once again, as illustrated. However, it can be shown from (4),(5) that any first order reflection mode beam (e.g. 40 a(X), +1 mode)created by diffraction at an SRG of an incident beam (e.g. 42 a(X), −1mode) which is itself a first order reflection mode beam created by anearlier diffraction of an original beam (e.g. 34(X)) at the same SRGwill revert to the direction of the original beam (e.g. φ(X), ±θ(X),which is the direction of propagation of 40 a(X)). Thus, propagationwithin the fold zone 14 is restricted to a diamond-like lattice, as canbe seen from the geometry of FIG. 9B. The beam labelled 42 ab(X) is asuperposition of a specular reflection beam created when 42 b(X) meetsthe fold SRG 54 and a −1 mode beam created when 40 a(X) meets the foldSRG at substantially the same location; the beam labelled 42 ab(X) is asuperposition of a specular reflection beam created when 40 a(X) meetsthe fold SRG 54 and a +1 mode beam created when 42 b(X) meets the foldSRG at substantially the same location (and so on).

The exit SRG and incoupling SRG 52, 56 are oriented with a relativeorientation angle A′ (which is the angle between their respectivegrating lines). At each of the exit regions, the version meeting thatregion is diffracted so that, in addition to a zero order reflectionmode beam propagating downwards in the direction φ′(X), ±θ′(X), a firstorder (+1) transmission mode beam 38(X) which propagates away from theoptical component 10 in an outward direction φ_(out)(X), θ_(out) (X)given by:sin θ_(out)(X)sin(A′+φ _(out)(X))=n sin θ′(X)sin(A′+φ′(X))  (6)

$\begin{matrix}{{\sin\;{\theta_{out}(X)}{\cos\left( {A^{\prime} + {\phi_{out}(X)}} \right)}} = {{n\;\sin\;{\theta^{\prime}(X)}{\cos\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)}} + \frac{\lambda}{d_{3}}}} & (7)\end{matrix}$

The output direction θ_(out)(X), φ_(out)(X) is that of the output beamsoutside of the waveguide (propagating in air). For a flat waveguide,equations (6), (7) hold both when the exit grating is on the front ofthe waveguide—in which case the output beams are first ordertransmission mode beams (as can be seen, equations (6), (7) correspondto the known transmission grating equations)—but also when the exitgrating is on the rear of the waveguide (as in FIG. 7F)—in which casethe output beams correspond to first order reflection mode beams which,upon initial reflection from the rear exit grating propagate in adirection θ′_(out)(X), φ′_(out)(X) within the optical component 10 givenby:n sin θ′_(out)(X)sin(A′+φ′ _(out)(X))=n sin θ′(X)sin(A′+φ′(X))  (6′)

$\begin{matrix}{{{n\;\sin\;{\theta_{out}^{\prime}(X)}\cos\;\left( {A^{\prime} + {\phi_{out}^{\prime}(X)}} \right)} = {{n\;\sin\;{\theta^{\prime}(X)}\cos\;\left( {A^{\prime} + {\phi^{\prime}(X)}} \right)} + \frac{\lambda}{d_{3}}}};} & \left( 7^{\prime} \right)\end{matrix}$these beams are then refracted at the front surface of the opticalcomponent, and thus exit the optical component in a direction θ_(in)(X), φ_(in)(X) given by Snell's law:sin θ_(out)(X)=n sin θ′_(out)(X)  (8)φ′_(out)(X)=φ_(out)(X)  (9)As will be apparent, the conditions of equations (6), (7) followstraight forwardly from (6′), (7′), (8) and (9). Note that suchrefraction at the front surface, whilst not readily visible in FIG. 7F,will nonetheless occur in the arrangement of FIG. 7F.It can be shown from the equations (2-7) that, whend=d ₁ =d ₃  (10)(that is, when the periods of the incoupling and exit SRGs 52, 56substantially match);d ₂ =d/(2 cos A);  (11)andA′=2A;  (12)then (θ_(out) (X), φ_(out)(X))=(θ_(in) (X), φ_(in) (X)).Moreover, when the condition

$\begin{matrix}{\sqrt{\left( {1 + {8\;\cos^{2}A}} \right)} > \frac{nd}{\lambda}} & (13)\end{matrix}$is met, no modes besides the above-mentioned first order and zero orderreflection modes are created by diffraction at the fold SRG 54. That is,no additional undesired beams are created in the fold zone when thiscriteria is met. The condition (13) is met for a large range of A, fromabout 0 to 70 degrees.

In other words, when these criteria are met, the exit SRG 56 effectivelyacts as an inverse to the incoupling SRG 52, reversing the effect of theincoupling SRG diffraction for each version of the beam with which itinteracts, thereby outputting what is effectively a two-dimensionallyexpanded version of that beam 34(X) having an area substantially that ofthe exit SRG 56 (>>D² and which, as noted, is independent of the imagingoptics 17) in the same direction as the original beam was inputted tothe component 10 so that the outwardly diffracted beams formsubstantially the same virtual image as the inwardly inputted beams butwhich is perceivable over a much larger area.

In the example of FIG. 9B, A≈45° i.e. so that the fold SRG and exit SRGs54, 56 are oriented at substantially 45 and 90 degrees to the incouplingSRG 52 respectively, with the grating period of the fold region

$d_{2} = {d/{\sqrt{2}.}}$However, this is only an example and, in fact, the overall efficiency ofthe display system is typically increased when A≧50°.

The above considers flat optical components, but a suitably curvedoptical component (that is, having a radius of curvature extendingsubstantially along the z direction) can be configured to function as aneffective lens such that the output beams 30(X) are and are no longer ashighly collimated and are not parallel, but have specific relativedirection and angular separations such that each traces back to a commonpoint of convergence—this is illustrated in FIG. 7G, in which the commonpoint of convergence is labelled Q. Moreover, when every image point isconsidered, the various points of convergence for all the differentactive image points lie in substantially the same plane, labelled 50,located a distance L from the eye 37 so that the eye 37 can focusaccordingly to perceive the whole image as if it were the distance Laway. This can be equivalently stated as the various output beamsforming substantially the same virtual version of the current displayimage as the corresponding input beams, but at the distance L from theeye 37 rather than at infinity. Curved optical components may beparticularly suitable for short-sighted eyes unable to properly focusdistant images.

Note, in general the “width” of the fold and exit zones does not have tobe their horizontal extent—in general, the width of a fold or exit zone14, 16 is that zone's extent in the general direction in which light iscoupled into the fold zone 14 from the incoupling zone 12 (which ishorizontal in the above examples, but more generally is a directionsubstantially perpendicular to the grating lines of the incoupling zone12).

As indicated, phase distortions caused by diffractive optical elementsimprinted on the surface of a waveguide—such as the SRGs 52, 54, 56—candegrade image quality in a display system of the kind described above.In accordance with the present disclosure, this can be mitigated byintroducing suitable height offsets (i.e. in a direction substantiallynormal to the surface on which they are present) of the optical elementsrelative to one other and relative to the blank surface of thewaveguide.

FIG. 12A shows a side view of a part of a first waveguide 10 c of oneembodiment, which is suitable for use in a display system of the kinddescribed above. The waveguide 10 c has a first diffractive opticalelement O1 (e.g. one of the incoupling, fold or exit SRGs 52, 54, 56)and a second diffractive optical element (e.g. another of theincoupling, fold or exit SRG 52, 54, 56) imprinted on its surface (forexample on the rear of the waveguide 10 c, from the perspective of theviewer). The gratings O1, O2 are separated by a blank surface region B,which—as in the waveguide of FIGS. 10A and 10B—causes light to changephase by φ0 upon reflection from the blank surface region B. The blankregion B could for instance be the region W of FIG. 3B between theincoupling and fold SRGs 52, 54, or the unlabelled region between thefold and exit SRGs 54, 56.

The optical elements have the same structure (in particular, the samedepths h1, h2≠h1) as those in FIGS. 10A and 10B, with the first opticalelement O1 causing light to change phase by φ1=φ0−Δφ1 upon reflectiontherefrom and the second optical element O2 causing light to changephase by φ2=φ0−Δφ2 upon reflection therefrom.

The depths h1, h2≠h1 are, in contrast to the apodized gratings of FIG.11A, substantially constant over the entire area of the gratings O1, O2respectively, right up to the edges of the gratings O1, O2.Alternatively the depths may vary as a function of position (x,y) i.e.as functions h1(x,y), h2(x,y), but nevertheless falls sharply to zero atthe edges of the grating i.e. with a significantly steeper gradient thanthe apodized edges of the regions A1, A2 in FIG. 11A.

Moreover, in contrast to the waveguides 10 a, 10 b of FIGS. 10A, 10B and11A, the optical elements O1 and O2 gratings are at offset height withrespect to the blank TIR surface. Each height offset is selected suchthat the additional optical path length introduced by that offsetsubstantially matches the phase difference between reflection from therespective grating region and TIR. The additional optical path length isthe product of the refractive index n of the waveguide 10 c and theadditional distance which light traverses as a result of the offset.

The gratings O1, O2 are offset by distances Δh1 and Δh2 in thez-direction (i.e. in a direction substantially normal to the surface onwhich they are imprinted) respectively. The expanded view at the top ofFIG. 12A shows this offset in detail: in contrast to FIG. 10A, the topsof the filling regions of O1 and O2 are not level with the blank surfaceportion B, but are offset from B by Δh1 and Δh2 respectively.

The offsets Δh1 and Δh2 substantially match Δφ1 and Δφ2 respectively.That is, each offset Δh1, Δh2 is such as to increase the length of theoptical path traversed by a ray R1, R2 reflected at the respectivegrating O1, O2 relative to a ray R0 reflected at the blank surfaceregion by an amount that compensates for the differences in the phasechanges caused by reflection at O1, B, O2. For the grating O1 (resp.O2), the offset 4 h 1 (resp. Δh2) is such as to increase the opticalpath length traversed by a ray R1 reflected at the first grating O1(resp. a ray R2 reflected at the second grating O2) relative to thattraversed by a ray R0 reflected at the blank surface B by an amount overwhich the phase of the phase of the ray R1 (resp. R2) changes bysubstantially Δφ1 (resp.≈Δφ2). The optical path length traversed by theray R1 reflected from the first grating O1 is thus increased relative tothat traversed by the ray R2 reflected from the second grating O2 by anamount over which the phase of the ray R1 changes by substantiallyΔφ1-Δφ2. Phase matching does not need to be completely accurate toachieve acceptable image quality: phase changes from gratings and theTIR will be angle and wavelength dependent which means that ‘fully’optimal performance is obtained only for one case; for others is itless-optimal but nonetheless acceptable in terms of final image quality.In practice the system will be designed to meet conflicting requirementsin accordance with normal design practice.

A plane 90 is shown, which is perpendicular to the plane of thewaveguide 10 c. As will be apparent, assuming the rays R1, R0, R2 are inphase with one another when they arrive at the plane 90 prior toreflection at O1, O2 and B respectively (at points P1, P0, P2respectively), when the offsets Δh1, Δh2 substantially match Δφ1, Δφ2respectively in the above described manner, the rays R1, R0, R2 willalso be substantially in phase with one another when they arrive at theplane 90 again (at points Q1, Q0, Q2 respectively) after beingreflected. This will be true for any such plane lying below the gratingsO1, O2 and above the surface opposite the gratings (in this case thefront of the waveguide 10 c).

The resulting phase distribution of reflected beams within the waveguide10 c will thus be flat (as shown in FIG. 12B), without any phase jumpsthat would cause unwanted diffractive beam spreading.

The height offsets can be effected during manufacture, whereby asubstrate from which the waveguide 10 c is made is processed so that thegratings O1, O2 have the desired height offsets Δh1, Δh2. The gratingoffset can be effected by an etching process, for example, so that theblank area is offset from the grating areas by the desired amount.

FIGS. 13A and 13B show side and plan views of a part of a secondwaveguide 10 d in another embodiment. In this example, first and secondoptical elements O1 are substantially contiguous e.g. separated by ablank region of no more than W_(max)≈100 μm, and in some cases no morethan 50 μm. For example, the optical elements O1, O2 could be theincoupling and fold SRGs 52, 54 of FIG. 3B, with W≦Wmax. The firstoptical element 10 d is offset relative to the second optical component10 d by an amount Δh′ which substantially matches Δφ1-Δφ2 (which is alsoequal substantially equal to Δh1-Δh2) i.e. the offset Δh′ is such as toincrease the optical path length traversed by a ray R1 reflected at thefirst grating O1 relative to a ray R2 reflected at the second grating O2by an amount over which the phase of the phase of the ray R1 changes bysubstantially Δφ1-Δφ2.

The expanded view at the top of FIG. 13 A shows a small blank region b(e.g. ≦Wmax) separating the gratings O1, O2. When the blank region bseparating the gratings O1, O2 is sufficiently small, the improvementsin image quality can generally be realized just by matching Δh′ toΔφ1-Δφ2 without having to worry about the offsets relative to the smallblank region b as the effects of b may be negligible.

A plane 90 is also shown in FIG. 13A, equivalent to that of 12A. For anysuch plane 90, when Δh′ is thus configured, rays R1, R2 which arrive atthe plane 90 (at points P1, P2) in phase with one another prior toreflection at O1, O2 will also be substantially in phase when theyarrive at the plane 90 again (at points Q1, Q2) following reflection.

FIG. 14 and FIG. 15 show simulated results for example grating designsboth with (FIG. 15) and without (FIG. 14) offset height. FIG. 14corresponds to the waveguide 10 a of FIGS. 10A and 10B, and FIG. 15 tothe first waveguide 10 c of FIG. 12A. The graphs labelled a) show asimulated phase distributions for each waveguide; the graphs labelled b)show the corresponding point spread functions (PSF), and c) thecorresponding modulation transfer functions (MTF).

The PSF describes the response of an imaging system to a point source orpoint object. In this case, the response is measured in term of anglewhich represents the extent to which beam de-collimation occurs withinthe waveguides i.e. beam spreading due to diffraction. As will beapparent, a narrower PSF means less de-collimation, and thus a sharperimage.

The MTF is a measure of the ability of an optical system to transfervarious levels of detail from object to image. A theoretical MTF of 1.0(or 100%) represents perfect contrast preservation (in practice, notachievable due to diffraction limits), whereas values less than thismean that more and more contrast is being lost—until an MTF of inpractice around 0.1 (or 10%) when separate lines cannot bedistinguished, peaks merge together etc.

As can be seen from FIGS. 14 and 15, with the height offset thewaveguide has both a narrower PSF and good MTF over a larger range,which is indicative of improved image quality.

It should be noted that light reflected from an optical element mayexperience a zero phase change i.e. the optical element may cause lightto change phase upon reflection by an amount which is zero. For theavoidance of doubt, it should be noted that, in the following, when astructure is described as a causing light to change phase uponreflection by an amount, that amount may or may not be zero.

Note that the above arrangement of the light engine 13 is just anexample. For example, an alternative light engine based on so-calledscanning can provide a single beam, the orientation of which is fastmodulated whilst simultaneously modulating its intensity and/or colour.As will be apparent, a virtual image can be simulated in this mannerthat is equivalent to a virtual image that would be created bycollimating light of a (real) image on a display with collimatingoptics.

Making an optical component which includes SRGs typically involves theuse of microfabrication techniques. Microfabrication refers to thefabrication of desired structures of micrometer scales and smaller.Microfabrication may involve etching of and/or deposition on asubstrate, to create the desired microstructure on the substrate.

Wet etching involves using a liquid etchant to selectively dislodgeparts of a substrate e.g. parts of a film deposited on a surface of aplate and/or parts of the surface of the plate itself. The etchantreacts chemically with the substrate e.g. plate/film to remove parts ofthe substrate e.g. plate/film that are exposed to the etchant. Theselective etching may be achieved by depositing a suitable protectivelayer on the substrate/film that exposes only parts of the substratee.g. plate/film to the chemical effects of etchant and protects theremaining parts from the chemical effects of the etchant. The protectivelayer may be formed of a photoresist or other protective mask layer.

Dry etching involves selectively exposing a substrate e.g. plate/film(e.g. using a similar photoresist mask) to a bombardment of energeticparticles to dislodge parts of the substrate e.g. plate/film that areexposed to the particles (sometimes referred to as “sputtering”). Anexample is ion beam etching in which parts are exposed to a beam ofions. Those exposed parts may be dislodged as a result of the ionschemically reacting with those parts to dislodge them (sometimesreferred to as “chemical sputtering”) and/or physically dislodging thoseparts due to their kinetic energy (sometimes referred to as “physicalsputtering”).

In contrast to etching, deposition—such as ion-beam deposition orimmersion-based deposition—involves applying material to rather thanremoving material from a substrate e.g. plate/film. As used herein, theterm “patterning a substrate's surface” or similar encompasses all suchetching of/deposition on a plate or film, and such etching of/depositionon a plate or film is said to impose structure on the substrate'ssurface.

Conventional techniques for making an optical component involve, forinstance, first coating a to-be patterned region of a master plate'ssurface (desired surface region) in a chromium layer or other protectivemask layer (e.g. another metallic layer). The master plate and filmconstitute a substrate. The mask layer is covered in a positivephotoresist. Positive photoresist means photoresist which becomesdevelopable when exposed to light i.e. photoresist which has acomposition such that those parts which have been exposed to light (andonly those parts) are soluble in a developing fluid used to develop thephotoresist following exposure. Light which forms a desired gratingpattern (grating structure)—created, for instance, using two-beam laserinterference to generate light which forms a grating structure in theform of an interference pattern—is then projected onto the photoresistso that only the photoresist at the locations of the light bands isexposed. The photoresist is then developed to remove the exposed parts,leaving selective parts of the mask layer visible (i.e. revealing onlyselective parts) and the remaining parts covered by the unexposedphotoresist at the locations of the dark fringes. The uncovered parts ofthe mask layer are then be removed using conventional etching techniquese.g. an initial wet etching or ion beam etching process which removesthe uncovered parts of the mask but not the parts covered by thephotoresist, and which does not substantially affect the plate itself.Etching of the plate itself—such as further wet etching or further ionbeam etching—is then performed, to transfer the pattern from the etchedmask layer to the substrate itself.

FIG. 17A shows another optical component 10′ which is similar in somerespects to the optical component 10 of FIGS. 3A and 3B, but with someimportant differences that will now be discussed. As illustrated, theother optical component 10′ has SRGs 52′ (incoupling), 54′ (fold), 56′(exit) similar to those of the optical component 10, with large gaps(>>100 μm) between them, including between the incoupling and fold SRGs52′, 54′. Because of this large spacing, in manufacturing the otheroptical component 10′, the laser interference exposure could be done,using a positive photoresist technique along the lines of that outlinedabove, simply by applying shadow masks of different shapes in front of amaster plate (substrate) during laser interference exposure.

This is illustrated in FIG. 17B, which shows a master plate 70′ from theside during a two-beam laser interference exposure process. The plate70′ is coated in a chromium layer 72′, which is itself coated inphotoresist 74′, which is positive photoresist. The plate 70′ and film72′ constitute a substrate. An interference pattern is created by theinterference of two laser beams 67 i, 67 ii. A shadow mask 69′ is usedto prevent the pattern from falling outside of a desired portion (e.g.that spanned by incoupling SRG 52′) of the substrate's surface so thatthe only photoresist which is exposed is the parts covering the desiredportion on which light bands of the interference pattern fall (exposedphotoresist is shown in black and labelled 74′e in FIG. 17B). This canthen be repeated for any other portions to be patterned (e.g. for thosespanned by 54′ and 56′). The positive photoresist can then be developedto remove the exposed parts 74′e, and the substrate patterned in themanner outlined above.

The shadow mask, however, causes distortion near the edges of the DOEareas. The distortion is due to light scattering, non-perfect contact ofshadow mask and the finite thickness of the shadow mask (whicheffectively blurs the pattern near its edge). Herein, non-uniformity ofa grating structure exhibited near its edges (of the type caused by suchshadowing during fabrication, or similar) is referred to as “edgedistortion”. Edge distortion is indicated by the label D in FIG. 17B.

When the photoresist is developed, the edge distortion becomes embodiedin the developed photoresist along with the grating structure, and as aresult is transferred to the surface of the plate 70′ when it comes toetching. As a result, the final optical component 10′ (which eithercomprises or is manufactured from the patterned plate) also exhibitscorresponding edge distortion as indicated by the dotted lines labelledD around the edges of the various DOE areas in FIG. 17A.

Moreover, as well as creating edge distortion, it is difficult toposition the shadow mask 69′ accurately when exposing the substrate inthis manner, and therefore it would be difficult to reduce the size ofthe gaps between the SRGs 52′, 54′ without risking overlap between theSRGs 52′, 54′.

Returning to FIG. 3B, in contrast to the other optical component 10′ ofFIG. 17A, the incoupling and fold zones 12, 14 of the optical component10 are substantially contiguous in that they are separated by at most anarrow border zone 18 which has a width W as measured along (that is,perpendicular to) a common border 19 that divides the border zone 18.That is, the incoupling and fold zones are separated by a small distanceW in width along a common border 18. Moreover, the incoupling, fold andexit SRGs 52,54, 56 of the optical component 10 are free from edgedistortion of the kind described above. It has been observed that thisconfiguration produces superior image quality to that of the otheroptical component 10′.

In particular, it has been observed that, when the separation W of theincoupling and fold regions 12, 14 along the common border 19 (the gap)is reduced to W≦W_(max) along the length of the common border 19 (thatis, provided the incoupling and fold zones are separated by no more thanW_(max) in width along the length of the common border 19)—whereW_(max)≈50 μm (micrometers)—an improvement in image quality can beobtained. In practice, the size of gap at which the improvement isobserved may have some dependence on the thickness of the waveguide. Forexample, for a waveguide having a thickness (extent in the z direction,as it is shown in the figures) of approximately 0.6 mm or less, adramatic improvement in image quality is observed when W_(max) isapproximately 50 μm or less. This particular case is illustrated in FIG.10, which shows curve of MTF (modular transfer function) drop asfunction of gap width in one case included for FIG. 18. The increase inMTF as the gap is reduced from 50 μm is immediately evident in FIG. 18.As is well known to persons skilled in the art, the modular transferfunction (MTF) is a measure of the ability of an optical system totransfer various levels of detail from object to image. An MTF of 1.0(or 100%) represents perfect contrast preservation, whereas values lessthan this mean that more and more contrast is being lost—until an MTF of0 (or 0%), where line pairs (a line pair is a sequence of one black lineand one white line) can no longer be distinguished at all. For a thickerwaveguide—e.g. of thickness approximately 1 mm, an improvement is stillexpected for a gap size of up to 100 μm.

The common border 19 of FIG. 3B is arcuate (substantially semi-circularin this example), with the incoupling and fold regions 12, 14 havingedges which are arcuate (in this case, substantially semi-circular)along the common border 19. The edge of incoupling region 12 issubstantially circular overall.

The disclosure recognizes that conventional microfabrication techniquesare ill suited to making the optical component 10 of FIG. 3B. Inparticular, existing techniques are ill-suited to making opticalcomponents exhibiting the requisite incoupling-fold zone separationW≦W_(max) and which are free of edge distortion whilst still accuratelymaintaining the desired angular orientation relationship between thevarious SRGs 52, 54, and 56 described above with reference to FIG. 9B.

A microfabrication process for making an optical component will now bedescribed with reference to FIG. 16. The process of FIG. 16 can be usedto.

As will become apparent in view of the following, the process of FIG. 16can be used to make optical components of the type shown in FIG. 3B withthe requisite small spacing between incoupling and fold zones, which arefree from edge distortion, and which moreover exhibit the desiredangular orientation to a high level of accuracy.

That is, this disclosure provides a novel interference lithographicmethod, which enables grating to be manufactured on the surface of anoptical component that are spaced apart from one another by 100micrometers or less. This is not achievable typically achievable withtraditional interference lithographic methods.

FIG. 16 shows on the left-hand side a flow chart for the process and onthe right-hand side, for each step of the process, plan and/or sideviews of an exemplary master plate 70 as appropriate to illustrate themanner in which the plate 70 is manipulated at that step. Each side viewis a cross-section taken along the dash-dotted line shown in thecorresponding plan view.

An upper part of the plate's surface is coated with a chromium film 72.The plate 70 and film 72 constitute a substrate, a desired surfaceregion of which (specifically, the surface region defined by thechromium layer 72 in this example), in performing the process, isselectively etched to create incoupling and fold SRGs 52, 54. Theincoupling SRG 52 is fabricated on a first portion 62 of the desiredsurface region (incoupling portion), and the fold SRG 54 on a seconddistinct (i.e. non-overlapping) and substantially contiguous portion 64of the desired surface region (fold portion) having the reducedseparation W≦W_(max) along the (intended) common border 19. For theoptical component 10 shown in FIGS. 3A and 3B, the desired regioncorresponds to the rear of the component's surface from the perspectiveof the wearer.

The final etched substrate constitutes an optical component which may beincorporated in a display system (e.g. the display system 2 of FIG. 1),or which may be for use as a production master for manufacturing furtheroptical components e.g. a mould for moulding such components frompolymer (or indeed which may be used for making such moulds), in whichcase the SRGs 52, 54 as fabricated on the substrate's surface aretransferred to (the rear of) those components by the manufacturing e.g.moulding process.

At step S4 of FIG. 16, the chromium layer 72 is coated in a negativephotoresist film 74—that is, photoresist which becomes undevelopablewhen exposed to light i.e. photoresist which has a composition such thatthose parts which have been exposed to light (and only those parts)become substantially insoluble in a developing fluid used to develop thephotoresist once exposed so that the exposed parts (and only the parts)remain post-development. This includes coating the incoupling portion 62which is ultimately intended to be patterned with the incoupling SRG 52,as well as the fold portion 64 ultimately intended to be patterned withthe fold SRG 54.

At step S6, an area substantially larger than and encompassing theincoupling portion 62 is exposed (shown in this example as a rectanglecontaining the desired circular area 62) to light which forms thedesired incoupling grating structure (i.e. that of SRG 52). By directingtwo laser beams 67 i, 67 ii to coincide in an interference arrangement,an interference pattern which forms the desired incoupling gratingstructure, having a grating period d when incident on the photoresist74, is created. The interference pattern comprises alternating light anddark bands, whereby only the parts of the photoresist on which the lightbands fall are exposed (exposed photoresist is shown in black andlabelled 70 e in FIG. 16); however, in contrast to positive photoresist,it is these exposed parts 70 e which become undevelopable whereas thenon-exposed parts in the locations of the dark bands remain developable.

A shadow mask 69 is used to restrict the interference pattern to thelarger area. The larger area is large enough not only to encompass theincoupling surface portion 62 but also such that all the edge distortionD created by the shadow mask lies outside of the incoupling portion 62(in general, it is sufficient for the wider area to be such there issubstantially no edge distortion in the vicinity of the intended commonborder 19, even if there is some edge distortion present elsewherearound the edge of the incoupling portion 62).

A dummy grating portion 63 is also exposed to the same (or a similar)interference pattern at the same time for reasons that will be discussedin due course.

The exposed portions 62, 63 can be practically of any shape or size butthe excess exposure resulting from possible other exposures must notreach any “active part” of the desired exposure portions (i.e. in theillustration aside S6, other exposures must not overlap the circularincoupling portion 62).

As an alternative to using masks, the interference pattern could beprojected over the whole of the desired surface region so that noshadowing effects are present on the desired surface region at all.

During the exposure step S6, the plate 70 is supported by a mechanicalclamping or other fixing method in an laser interference exposure setup(exposure system) not shown in FIG. 16 to hold it steady relative to theexposure system (in particular, relative to the beams 67 i, 67 ii)whilst the exposure takes place. After step S6, the master plate 70 isunloaded from the laser interference exposure setup.

At step S8, the unloaded plate 70 is exposed to light 65 ofsubstantially uniform intensity, but with photo mask 80 in place toexpose photoresist and thus avoid photoresist development from areasoutside the incoupling and dummy grating areas 62, 63. That is, photomask 80 on the incoupling portion 62 and the dummy region 63 are used toprevent exposure of the portions 62, 63 to the uniform light 65. Thus,uniform light 65 is projected over the entirety of the desired surfaceregion but for the incoupling and dummy portions (as these are coveredby the photo mask 80) so that all of the photoresist other than thatcovering the incoupling and dummy portions 62, 63 becomes undevelopablethroughout. It is thus the photo mask which define the portions 62, 63(i.e. the portions 62, 63 have the same size and shape as thecorresponding photomask 80 used to protect those portions), and not theshadow masks used in S6. A mask aligner is used to position the photomask 80 accurately on correct position on the substrate. The maskaligner has components (e.g. ultraviolet-lamp, optics etc.) forgenerating uniform light for exposure and the mechanics for positioningthe photomask 80 to the correct position.

As will be apparent, the only photoresist to retain any record of thegrating structure(s) as projected at S6 is that which covers theincoupling and dummy portions—outside of those portions, all record ofthe grating structure(s) is intentionally destroyed. The entirelyexposed photoresist outside of the incoupling and dummy portions 62, 63includes all the parts of the photoresist that were subjected to theedge distortion D, thus completely removing any record of the edgedistortion from the photoresist. Due to the nature of the process, thereis virtually no distortion to the grating pattern.

At step S10, the photoresist is developed to embody the incoupling SRGgrating structure by removing only those parts of that photoresist thathave not been exposed to light using a developing fluid. All theexposed, undevelopable photoresist 74 e is left substantially unchangedby the development of step S10. As illustrated in the figures to theright of S10 in FIG. 16, substantially no photoresist outside of theportions 62, 63 is removed in step S10; the only removed photoresist islines of unexposed photoresist in the incoupling and dummy portions 62,63 corresponding to the locations of the dark bands of the interferencepattern as projected on the photoresist at S6.

At step S11, a chromium etching procedure is performed to etch thechromium layer 72 (but not the plate 70 itself) with the incoupling SRGpattern, such as dry etching of the chrome hard mask 72. In etching stepS11, the photoresist serves as an etching mask to restrict etching ofthe chromium layer 72 to the incoupling and dummy grating surfaceportions only, whereby structure is imposed from the photoresist to theincoupling and dummy portions 62, 63. However, the exposed, undevelopedphotoresist 74 e outside of the portions 62, 63 inhibits etching outsideof those portions 62, 63 so that no structure is imposed on the chromium72 outside of those portions 9 (i.e. outside of those portions, thechromium is substantially unchanged).

Once the chromium 72 has been etched thus, the exposed photoresist 74 eis removed (S12) and the chromium 72 recoated with fresh, unexposednegative photoresist 74 (S13).

As indicated above, the relative orientation angle between incouplingand fold SRGs is intended to be A as defined in equation (11) above andshown in FIG. 9B (with the incoupling and exit SRGs having a relativeorientation angle 2A, as per equation (12)). This can be achieved byre-loading the plate 70 in the same exposure system (previously used atS6) supported again by the same mechanical clamps or other fixingmethod, and rotating the plate 70 by an amount that matches A relativeto the exposure system so that any subsequently projected pattern isoriented to the original incoupling SRG pattern by A (S14). By using asuitable drive mechanism, it is possible to achieve highly accuraterotation of the plate 70.

However, due to inaccuracy of mechanical stoppers, the position of theplate 70 is not accurately the same as in step S6. This is illustratedin the plan view aside step S14 of FIG. 16, in which an angle α is shownto denote slight rotation relative to the plate's initial orientation atthe previous exposure step S6 caused by unloading/reloading of the plate70.

For this reason, prior to rotating the plate 70 at S14, the offset αbetween the plate position in S6 and S14 is first measured. Themeasurement is done using a moiré pattern 81. The moiré pattern 81changes when the plate is rotated and this can be used to measure theangle of the plate with better than 0.001 degrees resolution.

To create the moiré pattern 81, the dummy grating portion is re-exposedto the same interference pattern it was exposed to at step S6 (or atleast an interference pattern having the same angular orientation), asshown on the right-hand side of FIG. 16. The moiré pattern is evidentnotwithstanding the presence of the photoresist atop the dummy grating.The moiré pattern is created as a result of the interaction between theinterference pattern and the dummy grating, and when the angularalignment is better than e.g. 0.01 degrees, has a fringespacing—typically of the order of few mm—and thus clearly visible whenthe offset α is about 5 thousandths of a degree, and which increases asα is reduced towards zero, becoming maximal (effectively infinite) uponα reaching zero. The fringe spacing is determined by the offset α and,conversely, can be used to measure α.

This leaves the photoresist atop the dummy grating partially exposed; aswill become apparent, this is inconsequential. Notably, the dummygrating portion 63 is sufficiently offset from the fold grating portion64 for the photoresist atop the fold grating portion to remain unexposedin creating the moiré pattern 81.

Once α has been measured, at step S16 the plate 70 is rotated from thatinitial orientation by an amount=A−α (thereby accounting for a in therotation) so that the plate 70 now has an orientation A relative to itsinitial position at S6 to a high level of accuracy.

At step S18, an area substantially larger than and encompassing the foldportion 64 is exposed (shown in this example as a rectangle containingthe desired area 64) again by directing two laser beams 67 i, 67 ii tocoincide in an interference arrangement, leaving the parts of thephotoresist on which light bands fall undevelopable in a mannerequivalent to S6 (but without any additional dummy grating area beingexposed). In S18, the interference pattern has a period d/(2 cos A) whenincident on the photoresist. A shadow mask 69 is again used to restrictthe interference pattern to this area, which is large enough not only toencompass the fold surface portion 64 but also such that all the edgedistortion D created by the shadow mask lies outside of the incouplingportion 62 (or at least clear of the common border 16).

Some or all of the photoresist atop the incoupling grating will likelybe exposed at S18, which is inconsequential as it has no effect on theincoupling pattern which has already been etched into the underlyingchromium 72.

All other areas except fold portion 64 are then exposed (S19) to uniformlight 65 with a suitable photo mask 80 in place to prevent exposure ofthe fold portion 64 (and only that portion) in a manner equivalent tostep S8. This leaves all the photoresist covering the incoupling portion62 (and also that covering an exit portion ultimately intended to beetched to form the exit grating 56) exposed and therefore undevelopable.The photoresist is then developed to remove only the unexposed parts(S20) in a manner equivalent to step S10, the chromium one again etchedto transfer the fold SRG pattern from the photoresist to the chromium,and the photoresist removed following etching (equivalent to S11-S12).The incoupling portion is protected by the exposed and thereforeundeveloped photoresist 70 e, thereby preserving the incoupling gratingpattern already etched into the chromium.

The use of photo mask 80 to define the incoupling and fold portionsenables the location of the DOE areas to be controlled far moreaccurately then when simply using shadow masks to define those areas (asin the positive photoresist technique outlined above). It thus becomespossible to reduce the separation of those portions to W≦W_(max) whilststill retaining separation of those portions (i.e. without the etchedpatterns overlapping).

Although not shown explicitly in FIG. 16, it will be apparent that thechromium covering the grating area ultimately intended for the exit SRG56 (vertically below the incoupling and fold SRGs 52, 54) is unaffectedby the etching of both 511 and S22 as in both of those steps it isprotected by undeveloped photoresist.

A similar process could be repeated to etch the desired fold gratingstructure into the chromium, again using a moiré pattern to achieve ahighly accurate angular orientation of 2 A between the incoupling andexit grating structures. The exit grating in the present configurationis relatively far away from the input grating. Thus input grating andexit grating can be exposed to the same photoresist layer with largeenough shadow masks to avoid edge distortions.

Once all three structures have been etched into the chromium, the plate70 itself is subject to an etching procedure (e.g. ion-beam etching) inwhich the chromium now serves as an etching mask, whereby the gratingstructures are transferred from the etched chromium 72 to the plate 70itself to form the desired incoupling, exit and fold SRGs 52, 54, 56 onthe plate itself with very good angular accuracy, narrow gap W≦W_(max)between SRgs 52, 54, and good quality edges free form edge distortion.

Note that the dummy grating pattern is not etched onto the plate itselfas it is not desired on the final optical component.

Once the plate itself has been etched, the chromium is removed and theplate 70, can e.g. be used in a display system of the kind shown in FIG.1, to mould further optical components, or indeed to make such moulds.

It has been demonstrated that, using the process of FIG. 16, substratescan be patterned, free from edge distortion, with the actual relativeorientation angle between the incoupling and fold zones 14, 16consistently being arccos(d₁/(2d₂)) (see equations 11, 12 above) and/orone half of the relative orientation angle between the incoupling andexit SRGs 12, 16 (see equation 13, above) to within ±one thousandth of adegree (as measured from a representative statistical population ofsubstrates fabricated using the present techniques). However twothousandths of a degree may be still acceptable angular error in somepractical contexts. It should be noted that the subject matter is notlimited to the particular exit pupil expansion configuration or gratingstructures but applies to other configurations as well. Moreover, whilstthe above is described with reference to diffraction gratings in theform of SRGs, the subject matter is not limited to diffraction gratingsand encompasses any structures which cause different phase changes.

In embodiments of the various aspects set out in the Summary section,the structure of the first portion may constitute a first diffractiongrating. The structure of the second portion may also a seconddiffraction grating.

The first grating may have a depth different from the second grating.

The first grating may have a depth which is substantially constant overthe entire first portion up to the edge of the first grating. The firstgrating may have a depth which is substantially constant over the entirefirst portion up to the edge of the first grating, and the secondgrating has a depth which is substantially constant over the entiresecond portion up to the edge of the second grating.

The structure of the first portion may constitute a first diffractiongrating and the structure of the second portion may be substantiallynon-diffractive. The first grating may have a depth which issubstantially constant over the entire first portion up to the edge ofthe first grating.

The first and second portions may be substantially contiguous.

The first and second portions may be separated by no more than 100micrometers in width along a common border, and optionally no more than50 micrometers in width along the common border.

A third portion of the same surface may have a structure which causeslight to change phase upon reflection from the third portion by a thirdamount different from the first amount, wherein the first and thirdportions are adjacent the second portion so that the second portionseparates the first and third portions, and wherein the third portion isoffset from the second portion by a distance which substantially matchesthe difference between the second amount and the third amount.

The structure of the first portion may constitute a first diffractiongrating, the structure of the third portion may constitute a seconddiffraction grating, and the structure of the second portion may besubstantially non-diffractive.

The structure of the first portion may constitute an incoupling gratingvia which said light is coupled into the waveguide from the display ofthe display system. The structure of the second portion may constitutean exit grating via which said light exits the waveguide onto the eye ofthe user. The structure of the second portion may constitute anintermediate grating configured to manipulate the spatial distributionof the light within the waveguide.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

The invention claimed is:
 1. A waveguide having a front and a rearsurface, the waveguide for a display system and arranged to guide lightfrom a light engine onto an eye of a user to make an image visible tothe user, the light guided through the waveguide by reflection at thefront and rear surfaces; wherein a first portion of the front or rearsurface has a structure which causes light to change phase uponreflection from the first portion by a first amount; wherein a secondportion of the same surface has a different structure which causes lightto change phase upon reflection from the second portion by a secondamount different from the first amount; and wherein the first portion isoffset from the second portion by a distance which substantially matchesthe difference between the second amount and the first amount.
 2. Awaveguide according to claim 1 wherein the structure of the firstportion constitutes a first diffraction grating.
 3. A waveguideaccording to claim 2 wherein the structure of the second portionconstitutes a second diffraction grating.
 4. A waveguide according toclaim 3 wherein the first grating has a depth different from the secondgrating.
 5. A waveguide according to claim 2 wherein the first gratinghas a depth which is substantially constant over the entire firstportion up to the edge of the first grating.
 6. A waveguide according toclaim 3 wherein the first grating has a depth which is substantiallyconstant over the entire first portion up to the edge of the firstgrating, and the second grating has a depth which is substantiallyconstant over the entire second portion up to the edge of the secondgrating.
 7. A waveguide according to claim 1 wherein the structure ofthe first portion constitutes a first diffraction grating and thestructure of the second portion is substantially non-diffractive.
 8. Awaveguide according to claim 7 wherein the first grating has a depthwhich is substantially constant over the entire first portion up to theedge of the first grating.
 9. A waveguide according to claim 1 whereinthe first and second portions are substantially contiguous.
 10. Awaveguide according to claim 9, wherein the first and second portionsare separated by no more than 100 micrometers in width along a commonborder, and optionally no more than 50 micrometers in width along thecommon border.
 11. A waveguide according to claim 1 wherein a thirdportion of the same surface has a structure which causes light to changephase upon reflection from the third portion by a third amount differentfrom the first amount, wherein the first and third portions are adjacentthe second portion so that the second portion separates the first andthird portions, and wherein the third portion is offset from the secondportion by a distance which substantially matches the difference betweenthe second amount and the third amount.
 12. A waveguide according toclaim 11 wherein the structure of the first portion constitutes a firstdiffraction grating, the structure of the third portion constitutes asecond diffraction grating, and the structure of the second portion issubstantially non-diffractive.
 13. An image display system comprising: alight engine configured to generate an image; a waveguide having a frontand a rear surface, the waveguide arranged to guide light from the lightengine onto an eye of a user to make the image visible to the user, thelight guided through the waveguide by reflection at the front and rearsurfaces, wherein a first portion of the front or rear surface has astructure which causes light to change phase upon reflection from thefirst portion by a first amount, wherein a second portion of the samesurface has a different structure which causes light to change phaseupon reflection from the second portion by a second amount differentfrom the first amount, and wherein the first portion is offset from thesecond portion by a distance which substantially matches the differencebetween the second amount and the first amount.
 14. A display systemaccording to claim 13 wherein the structure of the first portionconstitutes an incoupling grating via which said light is coupled intothe waveguide from the display.
 15. A display system according to claim13 wherein the structure of the second portion constitutes an exitgrating via which said light exits the waveguide onto the eye of theuser.
 16. A display system according to claim 13 wherein the structureof the second portion constitutes an intermediate grating configured tomanipulate the spatial distribution of the light within the waveguide.17. A wearable image display system comprising: a headpiece; a lightengine mounted on the headpiece and configured to generate an image; awaveguide located forward of an eye of a wearer in use, the waveguidehaving a front and a rear surface, the waveguide arranged to guide lightfrom the light engine onto the eye of the wearer to make the imagevisible to the wearer, the light guided through the waveguide byreflection at the front and rear surfaces, wherein a first portion ofthe front or rear surface has a structure which causes light to changephase upon reflection from the first portion by a first amount, whereina second portion of the same surface has a different structure whichcauses light to change phase upon reflection from the second portion bya second amount different from the first amount, and wherein the firstportion is offset from the second portion by a distance whichsubstantially matches the difference between the second amount and thefirst amount.
 18. A display system according to claim 17 wherein thestructure of the first portion constitutes an incoupling grating viawhich said light is coupled into the waveguide from the display.
 19. Adisplay system according to claim 17 wherein the structure of the secondportion constitutes an exit grating via which said light exits thewaveguide onto the eye of the user.
 20. A display system according toclaim 17 wherein the structure of the second portion constitutes anintermediate grating configured to manipulate the spatial distributionof the light within the waveguide.